[LeetCode 274] H-index

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's h-index.

According to the definition of h-index on Wikipedia: "A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h citations each."

For example, given citations = [3, 0, 6, 1, 5], which means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 citations respectively. Since the researcher has 3 papers with at least 3 citations each and the remaining two with no more than 3 citations each, his h-index is 3.

Note: If there are several possible values for h, the maximum one is taken as the h-index.

Hint:

1. An easy approach is to sort the array first.
2. What are the possible values of h-index?
3. A faster approach is to use extra space.

solution:

1. O(nlog(n)), sort array, use definition to find maximum index.

2. use extra space, O(n). counting sort

public class Solution {
    public int hIndex(int[] citations) {
        if(citations.length <=0) return 0;
        int res = 0;
        int len = citations.length;
        int[] countarray = new int[len+1];
        for(int i=0;i=len) {
                countarray[len]++;
            }else {
                countarray[citations[i]]++;
            }
        }
        if(countarray[len]>=len) return len;
        for(int i=len-1;i>=0;i--){
            countarray[i] = countarray[i] + countarray[i+1];
            if(countarray[i]>=i) return i;
        }
        return res;
    }
}